<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title> 
    <style>
        input{
            color: #999;
        }
    </style>
</head>
<body>
    <input type="text" value="手机">
    <script>
        //1.获取事件
        var text=document.querySelector('input');
        //2.注册事件 获取焦点事件 onfucus
        text.onfocus=function(){
            if(this.value==='手机'){
                this.value='';
            }
            //获得焦点需要把文本框里面的文字颜色变黑
            this.style.color="#000";
        }
         //3.注册事件 失去焦点事件 onblur
        text.onblur=function(){
            if(this.value===""){
                this.value="手机";
            }
            this.style.color="#999";
        }
    </script>
</body>
</html>